Method for determining a number of items of encoded data that can be stored in a memory, and method for controlling sizes of items of encoded image data to be stored in a memory

ABSTRACT

Methods are provided for an apparatus which encodes items of data by variable-length coding and which stores the encoded items of data in a memory, wherein a size of one of the items of encoded data is variable by varying a value of a control parameter of the variable-length coding. One method includes: determining an amount of available memory space in the memory; determining a value of the control parameter for the variable-length coding of the items of data; and determining a number of items of encoded data that can be stored in the available memory space, based on an upper limit of the size of one of the items of encoded data at the determined value of the control parameter.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for determining the amount ofencoded data that can be stored in a known memory space, that is, in amemory space having a known storage capacity, as well as to a method forcontrolling the size of encoded data.

2. Description of the Related Art

JPEG is a common method for encoding still images and is specified inISO 10918-1 JPEG International Standard/ITU-T Recommendation T.81(“Digital Compression Coding of Continuous-Tone Still Images”), which isincorporated by reference herein. This reference is hereinafter referredto as the “JPEG specification”. This standard is reproduced inPennebaker and Mitchell, JPEG: Still Image Data Compression Standard,New York, Van Nostrand Reinhold, 1993, which is also incorporated byreference herein.

Most digital still cameras employ the so-called JPEG Baseline Codingtechnique, in which a still image is partitioned into 8×8 blocks, eachof which is transformed into the frequency domain by DCT (DiscreteCosine Transform), quantized, and variable-length encoded using Huffmancodes.

In a digital camera employing JPEG Baseline Coding, it is difficult topredict the remaining number of photographs that can be recorded in agiven memory space due to the unpredictable file sizes of the imagefiles derived from the photographs. Some cameras display a number thatindicates a remaining number of images that can be recorded in aremaining memory space, and control the file sizes of subsequentlyobtained images, so that the displayed number of JPEG images can bestored in the remaining memory space. A well-known method forcontrolling the file size of a variable-length coded image is theiterative optimization of the quantization tables. A softwareimplementation of this method is disclosed in Japanese laid open patentapplication No. H02-100487, whereas a purely hardware implementation ofthis method using multiple bit-counting sections is disclosed in U.S.Pat. No. 5,146,324. Both the software and hardware implementationsrequire extensive processing time and consume excessive battery power.

It has also been suggested to calculate and display the remaining numberof photographs that can be obtained with a digital still camera basedupon statistical models of the file size variation as described inUS2001/0000969A1 or in Japanese laid open patent application No.H05-110789. These methods rely on the average file size and itsstatistical variation only, and thus are not considerably reliable.

SUMMARY OF THE INVENTION

Given a variable-length method of encoding digital data into encodedfiles which has a control parameter allowing file size (or rate)control, the present invention provides a reliable method of predictingthe number of items of encoded data that can be recorded in a givenmemory space that takes into account the pre-selected control parameter,and the present invention provides an easy and non-iterative orsemi-iterative (meaning only one trial) method of controlling the filesize of encoded data in order to record a given number of items of theencoded data in a given memory space.

In particular, the present invention is provided in the context oftransform-coding, which is defined here to mean an encoding method inwhich input data to be encoded is transformed using some transformationmapping, and in which the transformed data is quantized usingquantization factors and then variable-length encoded, wherein thequantization factors can be adjusted to control the size or rate of theencoded data. Input data need not necessarily be raw data, but may beany pre-processed data. In particular, difference data between blocksbelonging to different frames within a movie may such an input data.Further, the invention is also directed to software, hardware, andnetworked implementations of the prediction and control methods and,finally, to a digital camera implementing at least one of these methods.

According to the present invention, a function is employed thatrepresents the theoretical upper limit of the size (or length) of theencoded data (or file), which is dependent on the control parameterallowing control of the file size. As used here, theoretical (ormathematical) upper limit means an upper limit of the file size thatcannot be exceeded by any file size with absolute certainty. By knowingthis function, the number of files recordable at a given value of thecontrol parameter is trivially given by the largest integer notexceeding the quotient of the memory space by the value of this functionfor the given control parameter. Alternatively, the knowledge of thisfunction can be combined with statistical prediction methods employing,e.g., the average file size and its variations, to furnish an advancedprediction method with reliability not possible with prior artstatistical methods.

It is also advantageous to control the file size of an encoded fileusing the inverse function, which enables that the encoded file to havea file size that is below a given limit with certainty. If the encodedfile size is found too be small, the control parameter can be adjustedby using standard prediction curves (for example, the file size versuscontrol parameter) once in order to achieve the desired file size belowthe given limit.

According to one aspect of the present invention, a method is providedfor an apparatus which encodes items of data by variable-length codingand which stores the encoded items of data in a memory, wherein a sizeof one of the items of encoded data is variable by varying a value of acontrol parameter of the variable-length coding. The method includes:determining an amount of available memory space in the memory;determining a value of the control parameter for the variable-lengthcoding of the items of data; and determining a number of items ofencoded data that can be stored in the available memory space, based onan upper limit of the size of one of the items of encoded data at thedetermined value of the control parameter.

According to another aspect of the present invention, a method isprovided for an apparatus which encodes items of data by variable-lengthcoding and which stores the encoded items of data in a memory, wherein asize of one of the items of encoded data is variable by varying a valueof a control parameter of the variable-length coding. The methodincludes determining an amount of available memory space in the memory;determining a number of items of encoded data to be stored in theavailable memory space; and adjusting a size of the each of the items ofencoded data to be stored in the available memory space based on anupper limit of the size of one of the items of encoded data at the setvalue of the control parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconjunction with the accompanying drawings, wherein like referencenumerals identify like elements, and wherein:

FIG. 1 is a schematic block diagram of a digital camera in which themethod in accordance with the invention can be applied;

FIG. 2 is a flowchart illustrating the process of scaling a quantizationtable for JPEG encoding;

FIG. 3 is a schematic illustration showing JPEG encoding for processinga block of image data;

FIG. 4 is a flowchart illustrating JPEG encoding;

FIG. 5 is a flowchart illustrating calculating the number of imagesrecordable in an available memory space;

FIG. 6 is a flowchart illustrating calculating a scale factor to performrate control encoded data in order to record a given number of items ofthe encoded data in a given memory space;

FIG. 7 is a flowchart illustrating a semi-iterative rate-control methodaccording to the present invention; and

FIG. 8 is a flowchart illustrating a network in which rate-control of avideo transmission is performed according to one aspect of the presentinvention.

DETAILED DESCRIPTION

A JPEG file may be formatted in the JFIF as defined in “JPEG FileInterchange Format Version 1.02 Sep. 1, 1992 C-Cube Microsystems” or inthe JPEG compressed version of Exif (Exchangeable image format)described in “Digital Still Camera Image File Format Proposal (Exif)Version 1.0 Mar. 24, 1995 JEIDA/Electronic Still Camera Working Group”or in the SPIFF format, defined in “ITU-T Rec. T.84, Annex F-StillPicture Interchange File Format, 1995”, all of which are incorporatedherein by reference in their entireties.

According to one embodiment of the present invention described below, amethod for predicting the number of items of encoded data that can berecorded in a given amount of memory space is applied to a digitalcamera that performs JPEG Baseline coding of still images in the JFIFformat or Exif format.

In JPEG Baseline coding, a (color) image includes, for example, oneluminance (Y) and two chrominance (Cb, Cr) matrices of 8-bit pixel data.Each of these matrices is divided into 8×8 blocks, and each block (afterlevel shifting) is transformed by DCT into transform coefficients F(u,v) consisting of DC and AC coefficients. These DCT coefficients arequantized with a quantization table Q(u, v) (≧1) by taking the integerportion of the quotient F(u, v)/Q(u, v). In general, differentquantization tables are used for luminance and chrominance coefficients(i.e., one quantization table for the luminance coefficients and onequantization table for the chrominance coefficients). The quantized DCcoefficients are differentially encoded with DC Huffman tables, whereasthe AC coefficients are zigzag scanned and then Huffman coded. In thisembodiment, the typical Huffman tables (referred to as typical Huffmancodes below) as given in appendix K of the JPEG specification are used.

JPEG Baseline coding allows the file size to be changed or controlled byvarying the quantization tables. A simple and frequently employed methodis scaling the quantization tables by varying a scale factor SF thatmultiplies the quantization tables Q₀(u,v) in K.1 or K.2 of appendix Kof the JPEG specification, which are henceforth referred to collectivelyas the default quantization tables, to define new, scaled quantizationtables corresponding respectively to different picture qualities. Thatis, a new quantization table Q(u, v) is obtained by formula (1):

Q(u,v)=max(INT(SF×Q ₀(u,v)),1),  (1)

where INT denotes the integer portion of a fractional number.

As can be seen from formula (1), the smaller the scale factor SF is, thefiner the quantization. If SF= 1/64, all quantization factors becomeone, and the file size of the JPEG image data becomes rather high,whereas if SF=1, the result is the default quantization tables Q₀(u,v),and the file size lessens.

More concretely, as shown in FIG. 1, a digital camera 1 according tothis embodiment includes a CCD 16 to capture an image obtained via anoptical unit 14, which may include one or more lens units, which may befixed or detachable. The captured image is sampled, converted to digitaldata (A/D converted) and pre-processed by an analog front end IC 18 toobtain digital image data. A digital signal processor (DSP) 10 convertsthe digital image data into Y-, Cb-, and Cr-component image data, whichis temporarily stored in an internal SDRAM 30 and is read out from theSDRAM 30 in 8×8 blocks of pixels to be JPEG-encoded. As shown in FIG. 1,the DSP 10 is connected to various components, including the analogfront end 18, the SDRAM 30, a memory interface (I/F) 22 for an internalflash memory 20, a memory I/F for an external (removable) memory 26, aUSB controller 36 and an LCD processor 34 via a central data bus 24, andthe DSP 10 controls the various components of the digital camera 1 viacontrol buses. The DSP 10 includes a CPU 11, which can execute firmwareroutines loaded from the internal flash memory 20, which is accessed viathe bus 24 and the memory interface (I/F) 22. In addition, the DSP 10includes an internal RAM 12. The DSP 10 includes a JPEG encoder(explained below with respect to FIGS. 3 and 4) including hardware- andsoftware-implemented components, and the DSP 10 performs JPEG encodingon the YCbCr digital image data. That is, the CPU 11 of the DSP 10executes JPEG-firmware to serve as a software portion of the JPEGencoder, and the DSP 10 also includes hardware components 13 of the JPEGencoder. The JPEG-encoded digital image data is stored in the removablememory 26, which is coupled to the data bus 24 through the memory I/F28. The removable memory 26 may be, for example, a removable flashmemory card, such as an xD card, a CompactFlash card, or an SD card. Thedigital camera 1 also includes a display, such as an LCD 32, which iscontrolled by an LCD processor 34. Still further, a USB connector 38 isprovided to connect the digital camera 1 to, for example, a hostcomputer, and the USB connector 38 is coupled to the data bus 24 via aUSB controller 36.

Moreover, the digital camera 1 includes an input unit 40 including, forexample, buttons and/or switches, operable by a user of the digitalcamera to input instructions to the digital camera 1. For example, theuser may operate the input unit 40 to: turn the digital camera 1 on andoff; to instruct the digital camera to enter a photographing mode, inwhich, for example, in which, for example, images are captured via theCCD and displayed on the LCD 30 as an electronic viewfinder and in whichimages can be captured via the CCD, processed and stored in theremovable memory 26; and to instruct the digital camera to enter aplayback mode, in which stored images are played back.

In addition, the user may operate by the input unit 40 to select animage quality at which captured images are stored. That is, as explainedabove, JPEG baseline coding allows the file size to be changed byvarying a scale factor SF that multiplies the quantization tables. Forexample, according to this embodiment of the present invention, thescale factor SF may be one of seven discrete values ranging from 1/64 to1 in non-uniform steps, namely 1/64, 1/16, ⅛, ⅙, ¼, ½ and 1. These scalefactors indicate the compression ratio used in the JPEG encoding, andthus indirectly indicate the image quality. The scale factor 1/64corresponds to a super-quality image; the scale factors in the range of1/16 to ⅙ correspond to a high-quality image; and the scale factors inthe range of ¼ to 1 correspond to a normal quality image. By operatingthe input unit 40, the user may select a desired quality of image, and ascale factor is set in the DSP 10 to JPEG encode the image according tothe desired image quality. If the user does not set a desired imagequality, a default image quality corresponding to a scale factor of ⅛,for example, is set. Moreover, it is also possible to set the imagequality based on a pre-photographing analysis of the complexity of theimage (for example, a higher image quality may be necessary to preservedetail in a more complex image). The seven discrete possible scalefactor values 1/64, 1/16, ⅛, ⅙, ¼, ½ and 1 are sufficient for practicalpurposes in a digital still camera. Nevertheless, the number of scalefactors is not limited to the seven discrete values 1/64, 1/16, ⅛, ⅙, ¼,½ and 1. For example, a greater or lesser number of discrete scalefactor values may be used. Alternatively, as will be clear from thedescription below, it is possible to provide a continuous range of scalefactor values (in contrast to the discrete possible scale factor valuesprovided in the present embodiment).

When a user sets a desired image quality, the scale factor SFcorresponding to the desired image quality is read out from the internalflash memory 20 and stored in the internal RAM 12 of the DSP 10, and acopy is stored in the SDRAM 30. More specifically, the SDRAM 30 stores,for example, information concerning which buttons of the input unit 40have been pressed, a current scale factor, system data, and so on, whichis read out from and written into the SDRAM 30 by the DSP 10 (the CPU)running firmware programs. The SDRAM 30 is also used by the DSP 10 forimage processing, to store, for example, the YCbCr data, as mentionedabove, and the JPEG file created by the JPEG processing.

When an image is captured and before JPEG encoding is performed on thecaptured image at the set scale factor, the DSP refers to the set scalefactor SF and executes a firmware routine read out from the internalflash memory 20, to generates a scaled quantization table Q(u, v) foreach of the luminance and chrominance data based on the defaultquantization tables Q₀(u, v) for the luminance and chrominance data byperforming the scaling process shown in FIG. 2. That is, in the scalingprocess, the DSP 10 calculates Q(u, v) by formula (1) based on the setscale factor SF (stored in the internal RAM 20) and a defaultquantization table Q₀ (u, v) (which has been read out from its storagelocation in the internal flash memory 20) to obtain a quantization tablescaled in accordance with the set scale factor SF (step S202). Thescaled quantization table is output from the DSP 10 and temporarilystored in the internal RAM 12 of the DSP 10 (step S204). The scalingprocess is performed twice (consecutively): once to scale the luminancequantization table and once to scale the chrominance quantization table.

As noted above, the DSP 10 includes a JPEG encoder includinghardware-implemented components 13 in FIG. 1 and software-implementedcomponents, which are implemented by the CPU 11 executing firmwareroutines. FIG. 3 illustrates the structure of the JPEG encoder, and theflowchart of FIG. 4 illustrates the process of JPEG-encoding an image.Only the elements of the JPEG encoder that are relevant to thediscussion of the present invention are illustrated, since there arenumerous well-known JPEG encoders.

In FIG. 3, element 50 represents the internal flash memory 20, whichstores the default quantization tables as well as typical Huffman codesfor the AC and DC coefficients. Element 51 represents the CPU 11, whichcalculates the scaled quantization tables as explained above, withrespect to FIG. 2. Moreover, element 52 represents the internal RAM 12,which stores the set scale factor, the scaled quantization tables, andother temporary values for use in JPEG encoding of an 8×8 block ofpixels. Elements 53-59 represent the hardware-implemented components ofthe JPEG encoder, which are designated by reference numeral 13 inFIG. 1. In more detail, the JPEG encoder includes a DCT unit 53, aquantization unit 54, a zigzag ordering unit 55, a DC encoder 56, an ACencoder 57, a bit counter 58, and a JPEG formatter 59.

Before JPEG encoding an image, the CPU 51 reads the default quantizationtables from the internal flash memory 50, scales the defaultquantization tables, and stores the scaled quantization tables in theinternal RAM 52, as described above. The CPU 51 reads the scaledquantization tables from the RAM 52 and writes them into an internalregister of the quantization unit 54. Then, the CPU 51 reads out thetypical Huffman codes (for AC and DC coefficients, for luminance andchrominance, as required), and the CPU 51 writes the Huffman codes intothe internal registers of the corresponding hardware blocks (that is,the DC encoder and the AC encoder) (step S401)

In the JPEG encoding, each of the Y, Cb and Cr matrices of the imagedata stored in the SDRAM 30 is divided into 8×8 blocks by the DSP 10,and the JPEG encoding is performed for each block in turn. Morespecifically, a given 8×8 block (of pixels) of image data is read outfrom the SDRAM 30 (or from the internal RAM 12) (step S402), and the DSPperforms (after level shifting) DCT on the level-shifted 8×8 block usingthe DCT unit 53 (step S403). The DC and AC coefficients obtained by theDCT are quantized by the quantization unit 54 using the relevant (forluminance or chrominance data) scaled quantization table written intothe internal register of the quantization unit 54 (step S404). Thequantized coefficients are zigzag scanned by zigzag ordering unit 55(step S405).

The AC coefficients, which have been ordered by the zigzag ordering unit55, are outputted to the AC encoder 57 and are variable-length coded (ACHuffman coding), The AC encoder 57 has a built-in bit calculator whichcalculates the bit-length of the AC coefficients in the encoded 8×8block by adding the length of each Huffman code and of the amplitude ofeach code together. (Step S4061) The AC encoder 57 outputs the bit countto the bit counter 58 (step S4062), and outputs the AC Huffman codes tothe JPEG formatter 59 (step S4063).

The DC coefficient, on the other hand, is outputted to the DC encoder56, which performs differential coding of the DC coefficient (stepS4071). The DC encoder 56 outputs the bit-length of the DC Huffman codeto the bit counter 58 (step S4072), and outputs the DC Huffman code tothe JPEG formatter 59 (step S4073). The term DC Huffman code encompassesthe Huffman code itself as well as the amplitude code.

At step S408, the DSP 10 waits until both the AC and DC Huffman codeshave been outputted to the JPEG formatter 59, and at step S409 the DSP10 checks whether AC and DC Huffman codes have been obtained for every8×8 block of pixels in the image being encoded. If all of the 8×8 blockshave not been processed, the process returns to step S402, and stepsS402 to S409 are repeated until AC and DC Huffman codes have beenobtained for every 8×8 block of pixels have been obtained for every 8×8block of pixels. The AC and DC Huffman codes are formatted in accordancewith the EXIF format by the JPEG formatter 59 (step S409). The JPEGformatter 59 adds various other codes to the AC and DC codes in an orderprescribed by the EXIF format. The added codes are inputted to the JPEGformatter 59 from the CPU 51 and include, for example, marker codes(e.g., Start of Image, or SOI), an RST (restart marker), the typicalHuffman codes, the employed (scaled) quantization tables), the date ofphotography, the name of the camera device, and so on. The JPEGformatter 59 also counts bit-lengths of the added codes and outputs thebit-lengths to the bit counter 58 (step S410), and the JPEG formatter 59outputs the formatted JPEG file portion by portion to the SDRAM (stepS411). At step S412, the DSP 10 checks whether all of the 8×8 blocksmaking up the image being encoded have been encoded. If all of theblocks have not been encoded, the process returns to step S402, toprocess the next 8×8 block. On the other hand, when all of the blockshave been encoded and outputted to the SDRAM, the process ends. When theoutputting of the JPEG file is completed, the JPEG file is transferredto the removable memory 26.

The bit counter 58 adds the received bit-lengths together and outputsthe sum periodically to the CPU 51. If the CPU has been programmed toperform rate control, the CPU may monitor the sum of the bit-lengths todetermine whether a target JPEG file length is likely to be exceeded. Inconventional JPEG encoders, the bit counter 58 may be used toiteratively refine the scale factor, which is not performed according tothe present invention. However, the bit counter 58 may be used in thealternative embodiment described below to perform a semi-iterativetechnique.

According to the present invention when the digital camera is turned onand is in the photographing mode, the DSP 10 performs a process todetermine the number of JPEG encoded images that can be stored in thememory space remaining in the removable memory 26, based on atheoretical upper limit for the file size of the JPEG encoded images.The theoretical upper limit for the file size of the JPEG encoded imagesis dependent on a control parameter (of the file size), namely, thescale factor SF in this embodiment.

In more detail, as is clear from the foregoing, a JPEG image fileincludes the coded DC and AC coefficients, as well as the other JPEGfile portion including, for example, the Huffman tables, thequantization tables, restart markers (RST), and a header portion with athumbnail image, and so on. According to the present invention, thetheoretical upper limit is determined as the sum of respectivetheoretical upper limits of the DC coefficient portion, the ACcoefficient portion, and the other JPEG file portion.

Using the rules of the JFIF and Exif specifications, it is ratherstraightforward to predict an upper limit for the code-length of thefile portion other than the DC and AC coefficients. Let h denote thetheoretical upper limit function for the bit-length of this JPEG fileportion. Although this portion contains the scaled quantization tablesthemselves, which depend on the scale factor SF, the variation incode-length at different scale factors is not large compared tovariations of the AC coefficient code sections. Therefore, apredetermined upper limit value will be used for the quantizationtables. Further, the thumbnail image itself is often encoded using JPEG.Although one could apply an upper limit function of this invention (seebelow) to the thumbnail image, prior art methods such as iterativefile-size control (described above) work well to encode the thumbnailimage into a given code-length frame, because the size of the thumbnailimage is very small and does not require much calculation effort.Therefore, the upper limit h is taken to be fixed and independent of thescale factor SF.

As for the DC coefficient portion, since DC Huffman codes are assignedto each DC coefficient difference separately (thus, there is no conceptof runlength-coding), a theoretical, maximum code-length d(SF) of the DCcoefficient portion can be determined as a simple sum of the longestpossible code-word for each such difference value. The maximumcode-length depends on the scale factor because the DC values must bequantized using a scaled quantization table, which may limit the maximumDC amplitude. For example, if the scaled luminance DC quantization valueis 16, then the maximum category of the DC coefficient differencebecomes 7 rather than the maximum category 11, whereby that the Huffmancode-length as well as the bit-size of the amplitude value lessens.

Finally, with respect to the AC coefficients, if the total number ofluminance 8×8 blocks is n1 and the total number of chrominance 8×8blocks is n2, and if l(SF) and c(SF) denote theoretical upper limits forthe bit-length for one 8×8 block of luminance and chrominance ACcoefficients, respectively, the theoretical upper limit for the ACcoefficient code-length is given by n1·l(SF)+n2·c(SF).

Thus, the theoretical upper limit f(SF) for the file-size of the JPEGfile is given by:

f(SF)=h+d(SF)+n1·l(SF)+n2·c(SF).  (2)

In formula (2), as explained above, h is a predetermined value that isindependent of scale factor SF, and which is stored in advance in theinternal flash memory 20. Moreover, the maximum code length d(SF) isdetermined as a simple sum of the longest possible code-word for eachdifference value, which is dependent on the scale factor SF. Since, asexplained above, in the present embodiment the scale factor SF may beone of seven discrete values ranging from 1/64 to 1 in non-uniformsteps, namely 1/64, 1/16, ⅛, ⅙, ¼, ½ and 1, there are only sevenpossible values for d(SF). Therefore, each of the seven possible valuesfor d(SF) may be stored in advance in the internal flash memory 20.Still further, with respect to the term n1·l(SF)+n2·c(SF), since in thepresent embodiment the scale factor SF may be one of seven discretevalues ranging from 1/64 to 1 in non-uniform steps, namely 1/64, 1/16,⅛, ⅙, ¼, ½ and 1, the values for the functions l(SF) and c(SF) aredetermined as having the values indicated by Table 1 (the method ofobtaining the values of Table 1 is described in more detail below). Thetheoretical upper limit f(SF) for the file-size of the JPEG file canthere by be determined for each of the seven discrete scale factors.

TABLE 1 Scale factor: SF 1/64 1/16 1/8 1/6 1/4 1/2 1/1 Luminance: l(SF)1134 956 812 715 654 517 429 in bits Chrominance: c(SF) 1071 797 670 603593 468 349 in bitsThus, a theoretical upper limit file size, which is dependent on acontrol parameter (namely, the scale factor SF in this embodiment) canbe determined for reach of the possible values for the control parameter(scale factor SF).

The values for each of d(SF), l(SF) and c(SF) may be predetermined andstored in the internal flash memory 20 for each of the seven possiblescale factor values in the present embodiment. Additionally (oralternatively), the seven possible values of f(SF) may be stored for agiven image size. However, if a continuous or nearly continuous range ofpotential scale factor values is provided, storing a value for each ofd(SF), l(SF) and c(SF) for each of the possible scale factor values maynot be economical. In this case, instead of storing actual values, theinternal flash memory 20 stores firmware routines that are executable bythe CPU 11 of the DSP 10 to perform calculations to determine the valuesof d(SF), l(SF) and c(SF), and then to determine a value of f(SF) forany input scale factor value in a certain range of interest.

Moreover, such values for determining the upper limit file size (valuesof h, d(SF), l(SF) and c(SF)) and/or values for the upper limit f(SF)itself may be predetermined and stored in the internal flash memory 20for particular types of images, such as medical images, astronomicalimages, graphical images, or other classes of images, which are encodedusing Huffman codes different from the typical Huffman codes.Alternatively, these values for the particular types of images can bedetermined using firmware routines, if the number of possible values forthe scale factor is large (for example, continuous, or nearlycontinuous, as noted above). For example, the digital camera 1 may becapable of taking different types of images in different modes(selectable by a user, or determined automatically based on an analysisof image content, for example). In this case, the upper limit value isdetermined based on the set mode and the set image quality.Alternatively, the digital camera 1 may be dedicated to a particularuse, such as astronomy or medical imaging. In this case, the digitalcamera would only be intended to take images of only a particular type,and the upper limit values for the encoded image size would only need tobe predetermined and stored beforehand in the internal flash memory 20(or, alternatively, calculated by the CPU 11) for the particular type ofimage, without, for example, storing the upper limit values based onencoding with the typical Huffman codes. The method of calculating upperlimit values according to the present invention for the typical Huffmancodes and for other Huffman codes will become apparent to one ofordinary skill in the art based on the description of the calculationmethod below.

Given a remaining amount of memory space M, the remaining number N ofimages taken with some scale factor SF is calculated by the DSP 10 asthe integer portion of the quotient of the memory space by thetheoretical upper limit:

N=INT(M/f(SF)).  (3)

When the digital camera is turned on and is in the photographing mode,the DSP 10 performs the process shown in FIG. 5 based on a firmwareroutine read from the internal flash memory 20 to determine the numberof JPEG encoded images that can be stored in the memory space remainingin the removable memory 26. That is, given inputs of the available(remaining) memory space M and the set scale factor SF, the CPU 11 inthe DSP 10 executes the calculation shown in formula 3 to determiningthe number of images N that can be stored at the set scale factor SF(step S502). The DSP 10 generates a signal corresponding to the numberN, and stores a value of the number N in the internal RAM 12 and in theSDRAM 30. Moreover, the DSP 10 causes the number N to be output to auser by, for example, outputting a signal corresponding to the number Nto the LCD processor 34, which controls the LCD 32 to display the number(step S504).

The DSP 10 determines the available memory space M and calculates thecalculation of the number of images N that can be stored in theavailable memory space (executes the process of FIG. 5) each time thedigital camera is caused to enter the photographing mode, and each timethat there is a change in the content of the removable memory 26 whilethe digital camera is in the photographing mode, for example, each timethat a new image is photographed, JPEG encoded, and stored in theremovable memory 26.

It is also possible, according to the present invention, to set thevalue of f(SF) in accordance with the available memory space M and anumber of images that a user desires to take. That is, according to thepresent invention another photographing mode may be provided in which auser may operate the input unit 40 to designate a desired number ofimages to be stored in the remaining memory space. Accordingly, as shownin FIG. 6, which illustrates a process executed by the DSP 10 based on afirmware routine read from the internal flash memory 20, given a numberof images N input by a user, the CPU 11 in the DSP 10 obtains the amountof available memory space M (step S602), and performs the calculation:

f(SF)≦M/N.  (4)

And determines the scale factor SF for the value of f(SF) necessary toguarantee that the desired number of images N can be stored in theavailable memory M (step S604). In order to calculate the scale factorSF, or image quality, necessary to fit the N JPEG images in theavailable memory space M, the inequality in (4) is iteratively solved instep S602 until an appropriate value for f(SF), and hence SF, is found.In the present, preferred embodiment f(SF) is defined only for a finitenumber of discrete values of SF, so that simple comparison loops or afinite case selection algorithm suffices to choose the appropriate scalefactor SF. This scale factor computation realizes a very simple ratecontrol method, in which the encoded output data fits within apredetermined memory space with absolute certainty. The CPU 11temporarily stores the obtained scale factor SF in the internal RAM 12(step S606). The obtained scale factor SF is used in the calculation ofthe scaled quantization tables for JPEG encoding, as explained abovewith respect to FIGS. 2-4.

It should be understood that the determination of the theoretical upperlimit value of the file size at a given scale factor performed accordingto formula (2) is not equivalent to a mere “brute force” determinationof the upper limit of the file size. More specifically, in formula (2),the largest contribution comes from the two AC coefficient terms. Acrude upper limit for the code-length of AC coefficients in an 8×8 blockcan be estimated (instead of using the functions l(SF) and c(SF)) asfollows: the AC coefficient code-length can not exceed 63 times themaximum code-length of the Huffman codes in the JPEG specification,which is 16 bits, plus the maximum size, which is 10 bits. Thus a “bruteforce” upper limit (in bits) for the luminance AC coefficients as wellas for the chrominance AC coefficients is given by

u=63×(16+10)=1638.  (5)

Compared with the values of the functions l(SF) and c(SF) (see Table 1),this limit is obviously too high. Furthermore, this number was derivedwith no reference to the quantization table, and therefore is completelyindependent of the scale factor. If this value u were used in place ofl(SF) and c(SF) in deriving f(SF), the calculation methods (3) and (4)would become useless for practical purposes. Indeed, the uselessness ofthe crude upper limit is a significant reason why upper limits have notbeen applied to rate-control and related topics in consumer electronicsarea. By contrast, the method represented by formula (3) and (4) abovecan easily be implemented in a digital camera, and the present inventionallows the selection of a plurality of image qualities and furtherenables a simple rate control mechanism (4).

The calculation method of the upper limit functions l(SF) and c(SF) forthe AC coefficients within the framework of JPEG Baseline coding schemeis described in more detail below. As noted above, although the present,preferred embodiment makes use of a finite number (namely, seven) ofpossible values for the scale factor, it is possible to provide acontinuous range of possible scale factor values. In the following,details that are easily verifiable by one of ordinary skill in the artof image coding have been omitted.

<Mathematical Background>

The set of all level-shifted image pixel data of an 8×8 block withvalues in the range [−2⁷,2⁷−1] of signed integers fills an off-centeredcube I=[−2⁷,2⁷−1]⁶⁴ in a 64-dimensional space. This cube is contained ina ball B around the coordinate origin with radius R=2⁷×√{square rootover (64)}=2¹⁰. Although the DCT is known in the literature as a 1-to-1mapping from the image domain into the frequency domain, it is easy tocheck that the DCT, when viewed as a linear mapping within the64-dimensional space, is an orthogonal transformation. Therefore, theball B is mapped onto itself and the cube I is mapped into anotherrotated cube J within the ball B. Any matrix of DCT coefficients F(u, v)lie in J. F(u, v) is denoted simply by F and called a configuration andJ is called the configuration space.

After quantization with the scaled quantization table in formula (1),the quantized AC coefficients D(k), k=1, . . . , 63, are ordered in azigzag scanning order and are represented by runlength r, size s, andamplitude a. In the following, the scaled quantization table is alsoexpressed in zigzag scanning order: Q(k)=Q(u, v). Huffman codesHuff(r,s) as exemplified in appendix K of the JPEG specification,henceforth Typical Huffman codes, are assigned to each symbol (r,s),whereas each amplitude a is encoded with s bits. Let len( . . . ) denotethe length in bits of the final code for a given DCT configuration orfor a sub-sequence AC coefficients. For a quantized AC coefficientD(k)=a with size s preceded by r zeros, len(0, 0, . . . , 0,D(k))=len(r,s)=len(Huff(r,s))+s. For a sequence of coefficients withsizes s₁, s₂, . . . , s_(l), the notation len(s₁, s₂, . . . , s_(l)) isused.

<Problem and Simplification>

In order to derive the upper limit functions l(SF) and c(SF), themathematical problem of calculating an upper limit of the code-lengthsfor all configurations in j for a given scaled quantization table Q(k)must be solved. In order to solve this problem, the problem issimplified in several steps.

First, note that an upper limit of the code-length for any and allconfigurations in B is certainly also an upper limit of code-length forall configurations within the original configuration space J as a subsetof B. Henceforth, J is replaced by B and all possible configurations inB are considered.

Second, note that in B, the maximum code-length for a given quantizationtable does not lessen if any one of the quantization table coefficientsis replaced by a smaller coefficient. This can be shown by noting thatif F is a configuration in B, then any configuration F′ whosecoefficient amplitudes are less than or equal to the corresponding onesin F will still lie in B. Given the scaled quantization table Q(k), thescaled quantization table Q(k) is replaced by a new quantization tableQ₂(k) defined by the largest integer power of 2 not exceeding Q(k),Q₂(k):=2^(C(k)), (where C(k)=[log₂ Q(k)] (“power-of-2 quantizationtable”). The default quantization tables in appendix K of the JPEGspecification fulfill the relation Q₀(l)/Q₀(k)<2 if l<k, which can beshown to imply the relation C(l)−C(k)≦1 if l<k for the selected rangebetween 1 and 1/64 for the scale factor. Note that in this case,0≦C(k)≦6.

Third, if F_(max) denotes a maximum code-length configuration in B forthe power-of-2 quantization table C(k), F_(max) is required here to beefficient, i.e., it is required that its coefficients be non-negativeand chosen to be as small as possible without changing the ACcoefficient code-length and staying fully within B. Thus, the DCcoefficient is set to zero. AC coefficients can be assumed to be eitherzero or integer powers of 2, since if an AC coefficient is negative, itcan be replaced by its absolute amplitude, and further since if acoefficient is not an integer power of 2, it can be replaced with thelargest integer power of 2 not exceeding the coefficient withoutaffecting the quantized value due to the power-of-2 feature of thequantization table. In particular, if an AC coefficient is zero afterquantization, it can be set to zero already before quantization. Ineffect, F_(max) can be assumed to lie in the discrete space B₂ of allconfigurations in B with a zero DC coefficient and in which each ACcoefficients is either zero or a positive integer power of 2. Theconfiguration space B can henceforth be restricted to B₂.

Any configuration F in B₂ can be characterized by the size S(k), k=1, .. . , 63, of the AC coefficients, where the size can be 1, . . . , 10 asbefore or 0, which is defined to represent a zero coefficient.Quantization of F amounts to reduce these sizes by the powers-of-2 C(k)of the quantization factors, so that the quantized sizes are given bys_(k)=max(S(k)−C(k),0).

Thus, by the steps outlined above, the problem has been reduced to thatof finding an upper limit of code-lengths for an efficient maximumcode-length configuration in B₂ when using the power-of-2 quantizationtable Q₂ (k). This upper limit is automatically an upper limit ofcode-length for all DCT configurations for the given scale factor.

<Calculation Method>

Let F_(R) be the reference configuration which has a zero DC coefficientand all AC coefficients of the same value 2⁷, i.e., S_(R)(k)=8, andwhich has quantized sizes s_(k)=S_(R)(k)−C(k)=8−C(k)≧2. For an arbitraryefficient configuration F in B₂ having sizes S(k) and quantized sizes s_(k)= S(k)−C(k), operations MINUS and PLUS are defined which togethertransform F_(R) into the target configuration F. During thetransformation, each position p=1, . . . , 63 is traced in ascendingzigzag scan order and operated by MINUS first, followed by PLUS:

-   MINUS: If S(p) is non-zero, smaller than 8, and preceded by r zeros,    then replace S_(R)(p−r), . . . , S_(R)(p)    0, . . . , 0, S(p). If S(p) is greater than or equal to 8 and    preceded by r>0 zeros, then replace S_(R)(p−r), . . . , S_(R)(p)    0, . . . , 0, S_(R)(p). If S(p) is non-zero and all subsequent    S(q)=0 (q>p), then replace S_(R)(q)    0 for all such q>p.-   PLUS: If S(p) is greater than 8 and not preceded by zeros, then    replace S_(R)(p)    S(p). If S(p) is greater than 8 and preceded by r>0 zeros, then,    after being operated by MINUS, replace S_(R)(p)    S(p).

Let δ be the local code-length difference, which is defined to be thechange in code-length per affected position for these operations: if anoperation replaces m quantized sizes t_(i) at positions p_(i) by t _(i),but leaves n quantized sizes u_(j) unaffected, then it is given byδ=δ(t_(i),u_(j))=(len(t_(i),u_(j))−len( t _(i),u_(j)))/m. Thecode-length difference δ is assigned to each of the m affected positionsp_(i). These m positions together yield the total change in code-lengththrough the operation. By carefully studying the lengths of the TypicalHuffman codes, it can be shown that the MINUS always inducesnon-positive local code-length differences, whereas the PLUS operationinduces non-negative ones. Let −δ_(M)<0 denote the local code-lengthdifference induced by MINUS operation, wherein the index M runs over thepositions p, sizes, runlengths, and any cases that must bedistinguished. Let α_(q)>0 be the local code-length gain induced by PLUSfor s _(q)=s_(q)+1, and similarly let β_(q)>0 be the local code-lengthgain for s _(q)=s_(q)+2.

Now let F=F_(max) be an efficient maximum code-length configuration inB₂. The code-length of F_(max) can be represented as a sum of thecode-length of F_(R) and the local code-length differences for eachposition affected by the operations MINUS and PLUS defined to transformF_(R) into F_(max). By using obvious notations for the indices, we havelen(F_(max))=len(F_(R))+A+B−Δ, wherein the sum A=Σ_(q)α_(q) counts thelocal code-length gains induced by the PLUS operation for unquantizedsize 9, B=Σ_(q)β_(q) counts those for unquantized size 10, and thenon-negative sum Δ=Σ_(M)δ_(M) has contributions from the MINUSoperation.

Let a and b be the numbers of the terms in A and B, respectively, i.e.of those positions having unquantized sizes 9 and 10, respectively. LetS_(max)(k) be the unquantized sizes of F_(max). Let p_(i), i=1, 2, . . ., a, and q_(j), j=1, 2, . . . , b, be positions in F_(max) withS_(max)(p_(i))=9 and S_(max)(q_(j))=10, respectively. Since F_(max) liesin the ball B of radius 2¹⁰, its coefficients satisfy

${{\sum\limits_{{S_{\max}{(k)}} > 0}2^{{2{S_{\max}{(k)}}} - 2}} < 2^{20}},$

which is equivalent to

${\sum\limits_{{{S_{\max}{(k)}} > 0},{k \neq p_{i}},q_{j}}2^{{2{S_{\max}{(k)}}} - 2}} < {\left( {64 - {4a} - {16b}} \right) \cdot {2^{14}.}}$

This gives the constraint a+4b<16, which restricts the number ofcategory 9 and 10 positions to 40 combinations of the pair (a,b). Inparticular, 0≦a≦15 and 0≦b≦3. Further, since the left hand side sumsover at most 63−a−b positions, at least 3a+15b positions must haveunquantized coefficients with size less than 8. Thus there are at least3a+15b positions associated with the local code-length losses in Δ.Since these local code-length losses have been generated by the MINUSoperations for the particular configuration F_(max), the total sum Δ ofthese losses is certainly not smaller than the sum of only the first3a+15b smallest local code-length losses for any and all possible MINUSoperations for any and all target configurations F. More precisely, letΔ{ } be the set of all local code-length losses δ_(M) for any and allMINUS operations defined for any and all possible target configurations.If we define the function Δ(n) to be the sum of first n smallest valuesin Δ{ }, then Δ≧Δ(3a+15b).

In a similar fashion, let A{ } be the set of all local code-length gainsα_(q) for any and all possible target configurations F, and likewise,let B{ } be the set of all local code-length gains β_(q) for any and allpossible target configurations. Define the function A(n) to be the sumof first n largest values in A{ }, and the function B(m) to be the sumof first m largest values in B{ }. Then, A≦A(a) and B≦B(b). Thus, theinequality len(F_(max))≦len(F_(R))+A(a)+B(b)−Δ(3a+15b) is obtained. Theterms other than len(F_(R)) on its right-hand side are certainly notgreater than the maximum of the same expression A(a)+B(b)−Δ(3a+15b) forall 40 possible combinations (a,b), so that

len(F_(max)) ≤ Λ, where$\Lambda = {{{len}\left( F_{R} \right)} + {\max\limits_{({a,b})}{\left\{ {{A(a)} + {B(b)} - {\Delta \left( {{3a} + {15b}} \right)}} \right\}.}}}$

This is the desired upper limit of the code-length for any configurationin B₂ when using the power-of-2 quantization table Q₂(k), which has beenshown to be an upper limit for all DCT configurations when using thegiven scaled quantization table. By employing heuristic arguments, thederived upper limit Λ can be improved to a slightly lower upper limit Λ.

The above calculation method can be applied to the luminance andchrominance AC coefficients to yield the upper limits in Table 1.

The calculation method described above can be applied to quantizationtables which are not scaled versions of the default quantization tables,as should be clear to one of ordinary skill in the art. Moreover, asmentioned above, any Huffman code tables may be employed; that is, thepresent invention is not limited to the typical Huffman codes. Forexample, the JPEG specification does not restrict the Huffman codes tobe the typical Huffman codes (from appendix K of the JPEGspecification), but rather, near-optimum Huffman codes adapted forspecial classes of images, such as medical images (of human tissues),graphical images, or astronomical images (which include a few whitespots for the stars and a dark, or nearly dark, remainder), may be used.For each of such classes, specific Huffman codes may be determinedbeforehand and stored in the internal flash memory. Using thecalculation method described above, for each of these different Huffmancodes, one may determine the upper limit values for the JPEG file sizesand store these values in the internal flash memory.

In this preferred embodiment described above, only the one scale factorSF for both luminance and chrominance coefficients is used as a singlecontrol parameter in order to simplify the description. Moreover, theembodiment described above uses only discrete values of the scale factorSF. However, in an alternative embodiment, more than one such controlparameter may be defined to enable finer control of the file size. Forexample, one may take different scale factors SF1 and SF2 for theluminance and the chrominance quantization tables, respectively, afurther, discrete control parameter assigned to different sets ofHuffman tables could be applied. Still further, only one image pixelsize (e.g., SXGA) is considered for this embodiment to simplify theillustration. However, it is understood that the invention is applicableeven if a plurality of image pixel sizes, e.g. VGA, SXGA, or even otheraspect ratios are present, since, as is clear from the descriptionabove, the calculation of the upper limit builds upon individualcalculations for the DC and AC coefficients, which can be applied to anyimage pixel size or aspect ratio as long as the JPEG file format is thesame.

The calculation of the number of images N that can be stored in theremaining memory space M is not limited to equation (3) set forth above.For example, let a(SF) be the average JPEG file size for the given imagepixel size and a given scale factor and let σ(SF) be the variation inthe file size according to an assumed statistical model such as theGaussian or Laplace distribution. Such statistical data can be obtainedby taking a large number of sample pictures. This statistical data maydepend on the picture taking device, e.g. the type of the camera used.

Instead of deriving N as in equation (3), one can define:

N=INT((M−f(SF))/a(SF))+1,  (6)

giving the inequality (N−1)·a(SF)+f(SF)≦M. Thus, the number of imagesthat can be stored in the available memory space is calculated based onthe average JPEG file size, except for last image, which is calculatedbased on the theoretical upper limit function. In this way the user isable to store the last JPEG file into the memory with high probability.This feature is very advantageous for digital cameras, since the userdoes not want to miss the last chance to capture an image before themedia card of the camera is full.

In order to ensure this feature with even higher probability, thevariation should be taken into account for the derivation of N. In moredetail, instead of assuming that individual images (except for the lastimage) have the average file size a(SF), the 3sigma(SF) variation may betaken into account. For example, assuming a Gaussian distribution of thefile size, the standard deviation is given by sigma(SF) and the averageby a(SF). Assuming statistically independent images, the average size ofN′=N−1 pictures is given by N′ times a(SF), whereas the variation growsonly by the square root of N′, so that the standard deviation is nowgiven by SQRT(N′) times sigma(SF). The square root growth of thedeviation is the same for a 3sigma variation. Thus, by adding to theaverage size of N′ images the 3sigma variation for N′ statisticallyindependent pictures, a formula that can be used to derive N is:

(N−1)·a(SF)+√{square root over (N−1)}·(3σ(SF))+f(SF)≦M.  (7)

If the number N is selected to satisfy this inequality (e.g. iterativelyor explicitly solving this inequality which is easily possible for thoseskilled in the art), there is almost absolute certainty that the lastJPEG file can be stored into the memory space M. The factor of 3 beforethe variation term σ(SF) ensures 3-sigma certainty. The square root ofN−1 before the variation reflects the assumption that the variation isof purely statistical nature and that two successively stored JPEGimages have no statistical relation with each other. On the other hand,if repetitive shooting in a digital camera is considered, then there isusually high statistical correlation between successive images, so thatone should linearly add the variation,

(N−1)·(a(SF)+3σ(SF))+f(SF)≦M,  (8)

from which one obtains the formula

N=INT((M−f(SF))/(a(SF)+3σ(SF)))+1.  (9)

Thus, formulas (7) and (8) can be solved to give an N-value (e.g.formula (9) for formula (8) or by iterative calculation methods forformula (7)) instead of using formula (3), and the resulting N-value canbe displayed to the user. The new N-value(s) is larger than that of (3)and is closer to an average situation of photographing, since theaverage values are used. i.e. the displayed new N-value is closer towhat the user can actually store in the available memory space. Onedisadvantage is that there may occur situations where the actuallystored images are less in number than the predicted and displayed newN-values. In this context, note that the N-value (according to any oneof formulas (3), (7), or (8)) is calculated whenever the content of thememory card is changed, in particular whenever a new image has just beenstored in the memory card. Thus, depending on the updated amount ofavailable memory space, these formulae might give the same number N asone image before instead of decreasing by 1. When formula (3) is used,such a situation occurs more often than when using the alternativeformulas, since usually the file size will be around the average filesize and seldom comes close to the upper limit.

Moreover, according to the present invention, inequality (4) can be usedto define a semi-iterative (only one repetition) rate-control method, asshown in FIG. 7, which illustrates a process executed by the CPU 11executing a firmware routine read from the internal flash memory 20.Given a prescribed number N of to-be-taken JPEG image files input by auser, the CPU 11 determines the amount of available memory space M (stepS702), and solves inequality (4) by employing a small iterative programor the like to obtain the smallest scale factor value SF₀, which willresult in the largest file size possible (step S704). As long as suchcomputation does not involve the JPEG encoder or a bit-countingmechanism, the processing time is negligibly small for this computation.The scale factor SF₀ is stored in the internal RAM 12 and is used toscale the quantization tables as explained above with respect to FIG. 2(step S706) and to JPEG encode input image data as explained above withrespect to FIG. 4 (step S708). The encoded data is however not output(step S212 of FIG. 4 is not performed), but rather only its code-lengthf₀ is counted by the bit counter 58 and saved temporarily in the RAM 12(step S710). The JPEG file size f₀ obtained in this manner yields inmost cases a value smaller than f(SF₀). By employing the functionalrelation a(SF) between the scale factor and the average JPEG file size,the smallest scale factor SF₁ is selected in such a way that theinequality

f ₀ /a(SF ₀)·a(SF ₁)≦f(SF ₀)  (10)

is still satisfied (step S712). This new scale factor SF₁ is thenapplied to scale the quantization tables using the process explainedabove with respect to FIG. 2 (step S714) and to encode the same inputimage data for a second time using the process explained above withrespect to FIG. 4 (step S716) to yield the final JPEG file (in thissecond encoding, the data is outputted from the JPEG formatter to theSDRAM; that is, step S212 of FIG. 4 is performed). From statisticalbehavior, it can be expected that the final size, f₁, will behave withrespect to f₀ like the average a(SF₁) does with respect to a(SF₀), sothat

f ₁ ≈f ₀ /a(SF ₀)·a(SF ₁)≦f(SF ₀)  (11)

is still satisfied with a high degree of probability. Thissemi-iterative rate control method is advantageous against prior artrate control methods, since it only requires one iteration instead ofpossibly many. The semi-iterative rate-control of the present inventioncan be easily implemented as firmware executed by the CPU 11, and can beused, for example, to obtain almost constant file size instead ofkeeping the file size just below a targeted value.

The present invention can be easily applied to motion picture codingschemes like MPEG4 or H.264, which are described in detail in “H.264 andMPEG-4 Video Compression” by Iain E. G. Richardson, John Wiley & SonsLtd., 2003, which is incorporated herein by reference. The I-VOP and theP-VOP in the MPEG4 video coding scheme both apply the DCT, quantization,zigzag scanning, and variable-length coding. In particular, thequantizer scale parameter QP can be changed in the range from 1 to 31 tocontrol the code-length of 8×8 blocks in I-VOP.

The method of calculating the upper limit function according to thepresent invention can be applied here, with QP being the controlparameter. Thus, in order to keep the code-length of the I-VOP below aprescribed bit-length M₀ that is set in the apparatus that performs theMPEG encoding and read out (determined) by a CPU of the apparatus, thebit-lengths the N₀ 8×8 blocks in the I-VOP are controlled. By employingan upper code-length limit function g(QP) of the 8×8 block codes independence on the QP, according to the present invention the smallestquantizer scale parameter QP₀ satisfying the inequality:

g(QP ₀)≦M ₀ /N ₀  (12)

is determined by a CPU executing a firmware routine in the apparatusthat performs the MPEG encoding (e.g., a video camera). By quantizingthe blocks with this quantizer scale parameter, the total I-VOPbit-length becomes certainly below M₀. This description has beensimplified by dispensing with a discussion of code portions other thanthose from the 8×8 blocks in the I-VOP, but those skilled in the artwill recognize from this description how individual code-lengths ofI-VOP or P-VOP in MPEG4 encoding can be controlled using the presentinvention even if some overhead is present.

Since the 8×8 blocks in MPEG4 are encoded using the same scheme as inthe JPEG Baseline coding, i.e. DCT, quantization, zigzag scanning, andvariable-length coding, the mathematical derivation of the functiong(QP) is performed along the same lines as the calculation of the upperlimit functions l(SF) and c(SF) as explained in detail above.

This control method for the MPEG4 is in fact a non-iterativerate-control method. A video camera generates video frames (I-VOP orP-VOP, for example) with a fixed frame rate of 30 fps (frame persecond), for example. Controlling the individual bit-lengths below M₀bits amounts to controlling the bit-rate of the video stream below 30M₀bps (bits per second).

The method of the present invention can be implemented in a networkedenvironment, as shown in FIG. 8. For example, a video camera 61 thatproduces MPEG4 video (encoded moving image data) using a known encodercan be controlled from a server 62 over a communication network 63, suchas the Internet, to continuously and in real time transmit MPEG4 videoimages from a transmission section of the video camera 61 at 30 fps tothe server via the Internet in a well-known manner. Depending on thecommunication traffic on the network, the server can instruct thenetwork video camera to change the image quality by using therate-control algorithm of the present invention by transmitting aninstruction via the Internet 63 to the video camera 61 from atransmission section of the server 62. For example, if communicationtraffic is high, it may be desirable to reduce the transmission ratefrom the video camera over the network. That is, the server transmitsinstructions regarding the transmission rate to the video camera 61 viathe network 63. For example, the server instructs the network videocamera to keep the transmission rate below z (bps). The video cameracalculates (by a CPU thereof executing a firmware routine) the maximumallowed code-length M₀ for individual frames by the equation M₀=z/30 (inunit of bits per frame). The video camera 61 then performs therate-control method described above with respect to inequality (12) tocontrol the bit-rate to be below the instructed transmission rate bycontrolling the total I-VOP bit-length to be below the value of M₀corresponding to the instructed transmission rate.

The rate-control method of the present invention need not be applied toevery frame of a movie or Motion JPEG file. Rather, it is advantageousto apply it only for a restricted number of frames. For example, it maybe applied to every 15th frame within the Motion JPEG movie with a 30frames per second frame-rate.

The present invention is not restricted to Huffman or Exp-Golomb entropycoding, and is also applicable to context-based adaptive variable lengthcoding (CAVLC), arithmetic coding, and other variable length codingschemes.

Moreover, the present invention is not restricted to Discrete CosineTransform, but applies for any other transformations like wavelettransformations and lapped orthogonal transformations. Indeed, thepresent invention does not require a transform coding to be present atall. The factors important for application of the present are thecontrol parameter(s) and variable length coding.

1. A method for an apparatus which encodes items of data byvariable-length coding and which stores the encoded items of data in amemory, wherein a size of one of the items of encoded data is variableby varying a value of a control parameter of the variable-length coding,the method comprising: determining an amount of available memory spacein the memory; determining a value of the control parameter for thevariable-length coding of the items of data; and determining a number ofitems of encoded data that can be stored in the available memory space,based on an upper limit of the size of one of the items of encoded dataat the determined value of the control parameter.
 2. The method of claim1, wherein the determining of the number of items of encoded data thatcan be stored in the available memory space comprises determining aratio of the amount of the available memory space to the upper limit ofthe size of one of the items of encoded data at the determined value ofthe control parameter.
 3. The method of claim 2, wherein the number ofitems of encoded data that can be stored in the available memory spaceis an integer that is not greater than the ratio of the amount of theavailable memory space to the upper limit of the size of one of theitems of encoded data at the determined value of the control parameter.4. The method according to claim 1, wherein the upper limit of the sizeof one of the items of encoded data is a function of the controlparameter.
 5. The method according to claim 4, wherein the apparatus isadapted to determine the control parameter to be one of a plurality ofdiscrete values, and wherein the upper limit of the size of one of theitems of encoded data is stored beforehand for each of the plurality ofdiscrete values of the control parameter.
 6. The method of claim 1,further comprising outputting to a user the number of items of encodeddata that can be stored in the available memory space.
 7. The method ofclaim 5, wherein the outputting to the user comprises displaying thenumber of items on a display of the apparatus.
 8. The method of claim 1,wherein the encoding of the items of data is JPEG encoding of items ofimage data.
 9. The method of claim 8, wherein the control parameter is ascale factor that scales quantization tables used to quantize DC and ACcoefficients resulting from discrete cosine transform in the JPEGencoding.
 10. The method of claim 9, wherein the upper limit of the sizeof one of the items of encoded data is a function of the controlparameter.
 11. An apparatus which encodes items of data byvariable-length coding and which stores the encoded items of data in amemory, wherein a size of one of the items of encoded data is variableby varying a value of a control parameter of the variable-length coding,the apparatus comprising: means for determining an amount of availablememory space in the memory; means for determining a value of the controlparameter for the variable-length coding of the items of data; and meansfor determining a number of items of encoded data that can be stored inthe available memory space, based on an upper limit of the size of oneof the items of encoded data at the determined value of the controlparameter.
 12. An apparatus which encodes items of data byvariable-length coding and which stores the encoded items of data in amemory, wherein a size of one of the items of encoded data is variableby varying a value of a control parameter of the variable-length coding,the apparatus comprising: a memory space determining section whichdetermines an amount of available memory space in the memory; a controlparameter determining section which determines a value of the controlparameter for the variable-length coding of the items of data; and adetermining section which determines a number of items of encoded datathat can be stored in the available memory space, based on an upperlimit of the size of one of the items of encoded data at the determinedvalue of the control parameter.
 13. A method for an apparatus whichencodes items of data by variable-length coding and which stores theencoded items of data in a memory, wherein a size of one of the items ofencoded data is variable by varying a value of a control parameter ofthe variable-length coding, the method comprising: determining an amountof available memory space in the memory; determining a number of itemsof data to be stored in the available memory space; and determining avalue of the control parameter, based on an upper limit of the size ofone of the items of encoded data at the determined value of the controlparameter, to set an encoded size of the each of the items of data to bestored in the available memory space.
 14. The method according to claim13, further comprising: encoding each of the items of data to have theadjusted size after encoding; and storing the items of encoded data inthe memory.
 15. The method according to claim 13, wherein the upperlimit of the size of one of the items of encoded data is a function ofthe control parameter.
 16. The method according to claim 15, wherein theapparatus is adapted to determine the control parameter to be one of aplurality of discrete values, and wherein the upper limit of the size ofone of the items of encoded data is stored beforehand for each of theplurality of discrete values of the control parameter.
 17. A method foran apparatus which encodes moving image data, which comprises aplurality of frames, each of which comprises a plurality of units,wherein the apparatus encodes each of the units by variable-lengthcoding to encode the frames of moving image data, wherein a size of oneof the units of encoded data is variable by varying a value of a controlparameter of the variable-length coding, the method comprising:determining a maximum bit length of one of the frames after encoding;and determining a value of the control parameter, based on an upperlimit of the size of one of the units of data after encoding at thedetermined value of the control parameter, such that the bit length ofthe one of the frames after encoding does not exceed the maximum bitlength.
 18. The method according to claim 17, wherein the apparatustransmits the encoded moving image data to a server via a network. 19.The method according to claim 18, further comprising supplying aninstruction indicating a maximum transmission rate of the encoded movingimage data from the server to the apparatus via the network; whereinsaid maximum bit length of one of the frames after encoding isdetermined based on the maximum transmission rate.
 20. A systemcomprising: an apparatus which encodes moving image data, whichcomprises a plurality of frames, each of which comprises a plurality ofunits, wherein the apparatus encodes each of the units byvariable-length coding to encode the frames of moving image data, andwherein a size of one of the units of encoded data is variable byvarying a value of a control parameter of the variable-length coding; anetwork; and a server which communicates with the apparatus via thenetwork, and which supplies an instruction indicating a maximumtransmission rate of the encoded moving image data from the server tothe apparatus via the network; wherein the apparatus comprises: amaximum bit length determining section which determines a maximum bitlength of one of the frames after encoding based on the maximumtransmission rate; and a control parameter determining section whichdetermines a value of the control parameter, based on an upper limit ofthe size of one of the units of data after encoding at the determinedvalue of the control parameter, such that the bit length of the one ofthe frames after encoding does not exceed the maximum bit length.